The present invention relates to a hydraulically operated continuously variable transmission including a hydraulic pump and a variable-displacement hydraulic motor which are interconnected by a closed hydraulic circuit.
There have been proposed various hydraulically operated continuously variable transmissions for use in automobiles, including a swash-plate axial-plunger hydraulic pump having a pump cylinder coupled to an input shaft and a plurality of pump plungers slidably disposed in respective cylinder holes defined in the pump cylinder in an annular pattern around an axis of rotation of the pump cylinder, the hydraulic pump having an outlet port, a swash-plate axial-plunger hydraulic motor having a motor cylinder coupled to an output shaft and a plurality of motor plungers slidably disposed in respective cylinder holes defined in the motor cylinder in an annular pattern around an axis of rotation of the motor cylinder, the hydraulic motor having an inlet port, and a closed hydraulic circuit interconnecting the hydraulic pump and the hydraulic motor. Power is transmitted by the transmission through relative rotation between the motor cylinder and the pump cylinder.
As disclosed in Japanese Patent Publications Nos. 32-7159 and 41-3208, for example, communication ports arranged at substantially equal angular intervals in communication with the respective cylinder holes in the motor cylinder are defined in the motor cylinder and open at an axial end face thereof in an annular pattern around the axis of rotation of the motor cylinder. An annular distribution ring is disposed eccentrically with respect to the axis of rotation of the motor cylinder and held in slidable contact with the end face of the motor cylinder. Oil discharged from the hydraulic pump is introduced into the distribution ring, whereas oil discharged from the motor cylinder is introduced around the distribution ring.
The outlet and inlet ports of the pump are brought into successive communication with the communication ports in the motor cylinder in response to relative rotation between the distribution ring and the motor cylinder for reciprocally moving the annularly arranged motor plungers.
The distribution ring which confronts the end face of the motor cylinder in partly overlapping relation to the communication ports undergoes a force tending to press the distribution ring against the end face of the motor cylinder under the hydraulic pressure in and outside the distribution ring and also a force tending to separate the distribution ring off the end face of the motor cylinder under the hydraulic pressure from the cylinder holes in an area where the distribution ring overlaps the communication ports.
The hydraulic pressure in the cylinder holes varies according to acceleration or deceleration. The center of the force tending to separate the distribution ring from the end face of the distribution ring is out of alignment with the center of the force tending to press the distribution ring against the end face of the motor cylinder. For these reasons, oil leakage is likely to occur, and the mutually sliding surfaces of the distribution ring and the end face of the motor cylinder are apt to suffer localized wear due to irregular pressures applied thereto.